A plasma display panel (hereinafter referred to simply as a PDP) allows achieving high definition display and a large-size screen, so that television receivers (TV) with a large screen having as great as 100 inches diagonal length can be commercialized by using the PDP. In recent years, use of the PDP in high-definition TV, which needs more than doubled scanning lines than conventional NTSC method, has progressed and the PDP free from lead (Pb) is commercialized in order to contribute environment protection.
The PDP is basically formed of a front panel and a rear panel. The front panel comprises the following elements:                a glass substrate made of sodium-borosilicate-based float glass;        display electrodes, formed of striped transparent electrodes and bus electrodes, formed on a principal surface of the glass substrate,                    a dielectric layer covering the display electrodes and working as a capacitor; and                        a protective layer made of magnesium oxide (MgO) and formed on the dielectric layer.        
The rear panel comprises the following elements:                a glass substrate;        striped address electrodes formed on a principal surface of the glass substrate,        a primary dielectric layer covering the address electrodes;        barrier ribs formed on the primary dielectric layer; and        phosphor layers formed between the respective barrier ribs and emitting light in red, green, and blue respectively.        
The front panel confronts the rear panel such that its surface mounted with the electrodes faces a surface mounted with the electrodes of the rear panel, and peripheries of both the panels are sealed airtightly to form a discharge space therebetween, and the discharge space is partitioned by the barrier ribs. The discharge space is filled with discharge gas of Ne and Xe at a pressure ranging from 55 kPa to 80 kPa. The PDP allows displaying a color video through this method: Voltages of video signals are selectively applied to the display electrodes for discharging, thereby producing ultra-violet rays, which excite the respective phosphor layers, so that colors in red, green, and blue are emitted, thereby achieving the display of a color video.
The bus electrodes of the display electrodes employ silver electrodes in order to maintain electrical conductivity, and the dielectric layer employs low-melting glass made of mainly lead oxide. However, in recent years, dielectric layers free from lead for contributing to environment protection have been disclosed in, e.g. patent documents 1, 2, 3, and 4.
In recent years, the number of high-definition TV receivers has increased, which requires the PDP to increase the number of scanning lines, and then the number of display electrodes should be increased, so that intervals between the respective display electrodes must be reduced. As a result, the silver electrode forming the display electrode diffuses a greater amount of silver ions into the dielectric layer and the glass substrate. The diffused silver ions undergo reducing action from alkaline metal ions contained in the dielectric layer and divalent tin ions contained in the glass substrate, thereby forming silver colloid. As a result, the dielectric layer and the glass substrate tend to be yellowed or browned more noisily, and yet, silver oxide having undergone the reducing action generates oxygen which incurs air bubbles in the dielectric layer.
The increase in the number of scanning lines thus incurs yellowing in the glass substrate more noisily as well as more air bubbles in the dielectric layer, and those problems degrade the picture quality as well as generate failures in insulation of the dielectric layer.    Patent Document 1: Unexamined Japanese Patent Application Publication No. 2003-128430    Patent Document 2: Unexamined Japanese Patent Application Publication No. 2002-053342    Patent Document 3: Unexamined Japanese Patent Application Publication No. 2001-045877    Patent Document 4: Unexamined Japanese Patent Application Publication No. H09-050769